Track-registration process

ABSTRACT

A pre-etching treatment of a track-registration film wherein the irradiated film is exposed to ultra-violet radiation immersed in oxygenated water whereby the damage tracks are sensitized while annealing of the damage tracks is prevented.

I United States Patent [151 3,6 Caputi et al. [451 May 9, 1972 54 TRACK-REGISTR ATIQN PROCESS Primary Examiner.lames W. Lawrence AssisranlExaminerDavis L. Willis [72] Inventors: g g Eg i m Attorney-Ivor J.James, Jr., Samuel E. Turner, Sam E. Laub, raw or easan o o a l Frank L.Neuhauser, Oscar B. Waddell and Joseph B. Forman [73] Assignee: GeneralElectric Company [22] Filed: Feb. 9, 1970 57 ABSTRACT [2]] Appl. No.:9,818 I l A pre-etching treatment of a track-registration film wherein U8 Cl 250/83 CD 250/83 PH 156/, the irradiated film is exposed toultra-violet radiation im- [511 "G0." 6 mersed in oxygenated waterwhereby the damage tracks are [58] Fieid of j "250) CD 83 PH sensitizedwhile annealing of the damage tracks is prevented.

References and 16 Claims, 4 Drawing Figures UNITED STATES PATENTS.LSOLbBb 3/l970 Sherwood ..259/83 CD 3.505.523 4/1970 Becker ..Z50/83 CDTrack- Registration Sheet Energetic Irradiate with Particles Damage TraEtch in Selective ck Solvent Rinse and Dry Track- Registration //0 Sheetlrrodiofe with Energetic Pariicles Submerge in Oxygenoted Liquid andExpose to Electromagnetic Radiation Etch in Selective Damage TrackSolvent Rinse and Dry Fig INVENTORS:

ROGER W. CAPUTI WAYNE T. CRAWFORD /ZZ H ATTORNEY TRACK-REGISTRATIONPROCESS BACKGROUND A known technique for making visible the damagetracks formed in a body by energetic particles is described by Price etal. in U.S. Pat. No. 3,303,085 and by R. L. Fleischer et al. in anarticle entitled Tracks of Charged Particles in Solids," Science, July23, 1965, Vol. 149, No. 3682.

This particle track-registration process uses a body which may be acrystalline solid such as mica or a non-crystalline material such asinorganic glass or an organic polymeric plastic. When the body isirradiated with energetic particles, such as alpha particles or fissionfragments, damage tracks are formed in the material by local alterationof the structure of the material along the paths of the particles. Thedamage tracks are made visible by subjecting the body to a solvent whichpreferentially attacks the altered material along the damage tracks.Individual tracks are visible under an optical microscope after thisetching" treatment. Prior to etching, the tracks are generally notdetectable.

A wide variety of commercial applications have been discovered for theseparticle track-registration processes and materials. For example, wherethe irradiated body is in the form of a thin sheet or film, the holeswill pass entirely through the sheet, the individual holes havingdiameters in the range of 5-l00,000 angstroms. The perforated sheetsthus formed have utility as membrane filters, as described in U.S. Pat.No. 3,303,085. These techniques and materials have also been founduseful in neutron radiography, as described in copending U.S. Pat.application Ser. Nos. 558,490, now U.S. Pat. No. 3,493,741 (filed June17, 1966) and 601,112, now U.S. Pat. No. 3,457,408 (filed Dec. 12,1966).

While the present particle track-registration techniques give excellentresults, further improvements are desirable. In the production offilters, for example, it is highly desirable that the holes orperforations (often called pores) be substantially cylindrical in shape.If the perforations are relatively wide at the sheet surface and narrownear the center of the sheet, particles in the solution being filteredmay clog the filter by partially entering the perforation and jamming atthe narrow zone. Where the etching solvent attacks the body of the sheetand the damage tracks at about the same rate, the resulting perforationswill have this undesirable non-cylindrical shape. Thus, it is of thehighest importance that the etchant selective- 1y dissolve the alteredmaterial along the damage tracks at a much higher rate than it dissolvesthe body of the sheet. Also, it is important for rapid commercialproduction of filter materials that the etchant dissolve the alteredmaterials at a high rate so that filters of very small pore size may beproduced.

A method of increasing the etching rate of the damage tracks withrespect to the etching rate of the bulk material is described by W. T.Crawford et al. in copending U.S. Pat. application Ser. No. 812,463(filed Apr. 1, 1969) now U.S. Pat. No. 3,612,871 wherein, afterirradiation with energetic particles but before etching, the material issubjected to radiation of wave lengths less than about 4,000 angstromsor with electrons in the presence of oxygen. However, well-known sourcesof ultra-violet radiation also produce significant amounts of infraredradiation. Thus, if this pre-etch treatment is performed in a gaseousatmosphere without infrared filtering, it is found that the sheetmaterial is unduly heated whereby the damage tracks tend to disappearbecause of annealing.

SUMMARY It is an object of the invention to provide pre-etch radiationtreatment of track-registration material without undue heating orannealing of the material.

This and other objects are achieved in accordance with the invention byexposing the irradiated track-registration material to ultra-violetradiation while the material is submerged in a liquid coolant containingoxygen such as aerated or oxygenated water or a hydrogen peroxidesolution. The liquid coolant acts both as an infrared filter and as aneffective heat sink, thus preventing overheating and resultant thermaldamage to the track-registration material. Also, it is found that bythis means the length of ultra-violet exposure can be reduced to lessthan one-third the length of exposure required in a gaseousoxygen-containing atmosphere. Furthermore, the need for expensiveinfrared filters is eliminated.

DRAWING The invention is described in greater detail hereinafter withreference to the accompanying drawing, wherein:

FIG. 1 is a flow diagram of a preferred track-registration processaccording to the invention;

FIG. 2 is a schematic illustration of an electromagnetic treatmentarrangement;

FIG. 3 is an enlarged schematic illustration of a portion of atrack-registration sheet after irradiation with energetic particles; and

FIG. 4 is an enlarged schematic illustration of the sheet of FIG. 3after treatment with the selective etching solution.

DETAILED DESCRIPTION Shown in FIG. 1 is a flow diagram illustrating thesteps in a preferred process according to this invention.

A sheet of track-registration material 10 is prepared or otherwiseobtained. As is further detailed below, a wide variety of materials maybe used. Optimum materials for a given set of circumstances may beselected, depending on handling conditions, type of particles to beregistered, etc.

The sheet of track-registration material is irradiated with energeticparticles to form damage tracks comprising altered material along theparticle paths or trajectories in the material.

The irradiated track-registration material 10 is then submerged in anoxygen-containing liquid 11 as shown in FIG. 2. The liquid 11 may be,for example, aerated or oxygenated water or a hydrogen peroxidesolution. The material 10 is submerged in liquid 11 about 1 cm., forexample, with the surface of the material parallel to the surface of theliquid.

Next, the submerged irradiated material is exposed to electromagneticradiation of wavelengths less than about 4,000 angstroms, preferably andconveniently middle, near ultraviolet and/or ultra-violet light, as froman ultra-violet light source 12 which may be an I-I3T7 mercury vaporlamp available from the General Electric Company. The lamp is preferablymounted in a parabolic reflector and may be spaced from the surface ofthe liquid in the order of 12 cm. The liquid 11 acts as an infraredfilter and as a heat sink to thus prevent thermal damage of thematerial. As is further discussed below, while the mechanism is notfully understood, this treatment permits much more rapid selectiveetching of the altered material along the damage tracks than is the casewhere this treatment is omitted. In addition, etched tracks formed afterthis treatment tend to be much more uniformly cylindrical in shape.Where this treatment is omitted, good etched tracks may be obtainableonly with extensive etching and the etched tracks tend to be conical,being wider at the material surface.

Next, the exposed, irradiated material is subjected to an etchingsolution which selectively attacks and dissolves the altered materialalong the tracks and enlarges the damage tracks to the desired diameter.As is further described below, a wide variety of etching solutions andconditions may be used, where suitable. While these solutions do attackthe body of the trackregistration material, they attack the alteredmaterial along the damage tracks at a much greater rate.

Finally, the material is removed from the etching solution, rinsed(generally with water), and dried.

The sheet is now ready for the intended ultimate use.

FIG. 3 and 4 schematically show the track-registration material afterirradiation with energetic particles and after etching, respectively.

As seen in FIG. 3, the track-registration material contains a pluralityof substantially invisible damage tracks 21 caused by the passage ofenergetic particles. As illustrated, the particles entered the materialat a variety of angles. Of course, the particles may be collimated, ifdesired, so that all enter at substantially the same angle. At this timethe tracks are substantially invisible.

FIG. 4 shows material 10 after the treatment with the radiationdescribed above in the presence of oxygen and after etching. I-Iolesentirely through the material, as at 22, are formed where high energyparticles passed entirely through the material. Lower energy particlesresult in substantially cylindrical pits such as those at 23.

Any suitable synthetic resins may be treated in the process of thisinvention. Typical synthetic resins include: polyester resins such aspolycarbonates, polyethylene terephthalate; cellulosics such ascellulose nitrate, and cellulose acetate; and mixtures thereof. Wherethe track-forming energetic particles are alpha particles, thecellulosics are preferred, while polycarbonates are preferred where thetracks are formed by relatively heavy fission fragments.

Any suitable source of energetic particles may be used. Typical sourcesinclude self-fissioning isotopes such as californium-252; isotopes whichdecay by the emission of alpha particles, such as radium-226, andamericium-241; materials such as uranium-235 which fission and emitfission fragments when irradiated with neutrons; materials such asboron-10 which emit alpha particles when irradiated with neutrons; andvarious atomic and sub-atomic particle accelerators.

Any suitable etchant may be used to enlarge the damage tracks to thedesired diameter. The etchant may be an aqueous or non-aqueous solutionof acidic or basic catalysts or redox agents, where suitable. Typicaletchants include sodium hydroxide, potassium hydroxide, lithiumhydroxide, chromic acid, potassium permanganate, ammonium hydroxide,potassium t-butoxide, tetrapropyl ammonium hydroxide, and mixturesthereof. While any suitable solution concentrations may be used, aconcentrated solution is generally preferred for more rapid trackenlargement. Similarly, while the etching solution may be used at anysuitable temperature, it is generally preferable to heat (but not boil)the solution to increase the rate of track enlargement. Good results areobtained for polycarbonate, for example, with about 6-normal solution ofsodium hydroxide at about 80 C., with stirring or agitation of thesolution during treatment. Where small pore diameters are desired or toallow sufficient etching time for adequate quality control, a lowertemperature may be desirable.

The sheet may be treated with the etching solution for any suitable timeperiod. Where the period is too short, the tracks may not have asufficient diameter for the intended use, while too long a treatingperiod may allow the solution to attack the body of the sheetexcessively. The optimum time period will vary, of course, dependingupon the sheet material, the etchant; desired pore size and the solutionconcentration and temperature. Typically, with a polycarbonate sheet,developed in a 6-normal sodium hydroxide at about 60 C., about 20minutes in the solution produces fully developed tracks having adiameter of about 0.5 microns.

As is further pointed out in the examples below, treating the particleirradiated sheet with electromagnetic radiation, for exampleultra-violet light, while the sheet is submerged in an oxygen-containingliquid in accordance with the invention is found to preferentiallygreatly increase the etching rate of the altered material along theparticle damage tracks in the material while the etching rate of thebulk material does not significantly increase.

The pores formed by etching the treated sheets have been found to besubstantially cylindrical while those formed in untreated sheets oftenare somewhat conical, being wider at the sheets surface. Also, it hasbeen found that pores having smaller initial diameters may be producedin the treated sheet. While it is not fully understood why this dramaticimprovement results from this intermediate treatment, it is thought thatthe underlying processes are related to photochemical reactions whichinvolve a radical species in the altered material and oxidationreactions thereof.

The invention is further illustrated by the following examples:

EXAMPLE I Irradiated polycarbonate material was exposed in air to theradiation from an H3T7 mercury vapor lamp at a distance 25 cm. for 4minutes. No infrared filter was used. When etched in 6M NaOH at C., itwas very apparent that bulk degradation of the polycarbonate had takenplace to a depth of about 5 microns. Tracks were evident in theundamaged portion of the material.

A control sample, which was not exposed to the radiation, but which wassimilarly etched exhibited no pores.

A third sample was submerged in an oxygen-containing liquid (in thisexample, plain tap water) and therein exposed to radiation from themercury vapor lamp for 3 minutes. After subsequent etching in 6M NaOi-lat 60 C. for 8 minutes, this sample contained pores 16.6 microns inlength.

EXAMPLE II An irradiated polycarbonate sample was divided into threeportions. The first portion was etched in 6M NaOH at 60 C. for 8 minutesimmediately after irradiation and without exposure to ultra-violetradiation. No pores were evident.

A second portion was submerged in oxygen saturated water and exposed toultra-violet radiation (as shown in FIG. 1) for 3 minutes. After asimilar etching treatment, pores having a length of 19 microns werefound.

A third portion received a 3 minute ultra-violet exposure whilesubmerged in partially degassed water. After a similar etchingtreatment, pores limited to a length of 14.5 microns were found.

EXAMPLE III Exposure Time (minutes) Track Length (microns) 19.1 16.610.5 6.7 (no tracks) cop-w An additional portion was processed in asimilar manner except that it was exposed to ultra-violet radiation for3 minutes while submerged in a 10% H 0 solution. Track length in thiscase was found to be 18.1 microns.

No bulk degradation of the polycarbonate material was found in thesamples of examples II and Ill.

We claim:

1. In a track-registration process wherein a trackregistration materialis irradiated with energetic particles which form tracks of alteredmaterial along their trajectories in the material and the irradiatedmaterial is etched with a reagent which preferentially attacks thealtered material along said tracks; the improvement wherein prior toetching said irradiated material is exposed to electromagnetic radiationhaving wavelengths less than about 4,000 angstroms while submerged in anoxygen-containing liquid.

2. The process according to claim 1 wherein said energetic particles arealpha particles and said track-registration material comprises cellulosematerials.

3. The process according to claim 1 wherein said energetic particles arefission fragments and said track-registration material comprises apolycarbonate resin.

4. The process according to claim 1 wherein said electromagneticradiation is ultra-violet radiation.

5. The process according to claim 4 wherein said ultraviolet radiationhas wavelengths in the region from about 3,600 to about 3,800 angstroms.

6. The process according to claim 1 wherein said electromagneticradiation is gamma radiation.

7. The process according to claim 1 wherein said electromagneticradiation is X-radiation.

8. A track registration process comprising the steps of irradiating atrack-registration material with energetic particles which form damagetracks along paths in said material traversed by said energeticparticles; submerging said irradiated material in an oxygen-containingliquid coolant; and exposing the irradiated material to electromagneticradiation having wavelengths less than about 4,000 angstroms.

9. The process according to claim 8 wherein said oxygencontaining liquidcoolant is hydrogen peroxide.

10. The process according to claim 8 wherein said energetic particlesare alpha particles and said track-registration materi al comprisescellulose materials.

11. The process according to claim 8 wherein said energetic particlesare fission fragments and said track-registration material comprises apolycarbonate resin.

12. The process according to claim 11 wherein after exposure to saidelectromagnetic radiation said irradiated material is immersed in anetching solution comprising about 6-normal sodium hydroxide at atemperature of from about 50 C. to about C. for about 5 to 20 minutes.

13. The process according to claim 8 wherein said electromagneticradiation is ultra-violet radiation.

14. The process according to claim 13 wherein said ultraviolet radiationhas wavelengths in the region from about 3,600 to about 3,800 angstroms.

15. The process according to claim 8 wherein said electromagneticradiation is gamma radiation.

16. The process according to claim 8 wherein said electromagneticradiation is X-radiation.

UNITED STATES PATENT OFFICE QERTEFICATE OF CORRECTlQN Patent No. 3,662,178 Dated May 9, 972

Inventor(s) Roger W. Caputi and Wayne T. Crawford It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Abstract, line 2, after "radiation" insert --while--. Column 3, line 57,after "etchant", the semi-colon should be a comma. Column 5, line 12,after "said" insert --irradiated--.

Signed and sealed this 3rd day of October 1972.

(SEAL) Attest:

EDWARD IVLFLEI'CHEJR ,JR.

ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents USCOMM-DC60376-5 59 fi us GOVERNMENT PRINTING OFFICE: 1969 0-366-331

2. The process according to claim 1 wherein said energetic particles are alpha particles and said track-registration material comprises cellulose materials.
 3. The process according to claim 1 wherein said energetic particles are fission fragments and said track-registration material comprises a polycarbonate resin.
 4. The process according to claim 1 wherein said electromagnetic radiation is ultra-violet radiation.
 5. The process according to claim 4 wherein said ultra-violet radiation has wavelengths in the region from about 3,600 to about 3,800 angstroms.
 6. The process according to claim 1 wherein said electromagnetic radiation is gamma radiation.
 7. The process according to claim 1 wherein said electromagnetic radiation is X-radiation.
 8. A track registration process comprising the steps of irradiating a track-registration material with energetic particles which form damage tracks along paths in said material traversed by said energetic particles; submerging said irradiated material in an oxygen-containing liquid coolant; and exposing the irradiated material to electromagnetic radiation having wavelengths less than about 4,000 angstroms.
 9. The process according to claim 8 wherein said oxygen-containing liquid coolant is hydrogen peroxide.
 10. The process according to claim 8 wherein said energetic particles are alpha particles and said track-registration material comprises cellulose materials.
 11. The process according to claim 8 wherein said energetic particles are fission fragments and said track-registration material comprises a polycarbonate resin.
 12. The process according to claim 11 wherein after exposure to said electromagnetic radiation said irradiated material is immersed in an etching solution comprising about 6-normal sodium hydroxide at a temperature of from about 50* C. to about 80* C. for about 5 to 20 minutes.
 13. The process according to claim 8 wherein said electromagnetic radiation is ultra-violet radiation.
 14. The process according to claim 13 wherein said ultra-violet radiation has wavelengths in the region from about 3,600 to about 3,800 angstroms.
 15. The process according to claim 8 wherein said electromagnetic radiation is gamma radiation.
 16. The process according to claim 8 wherein said electromagnetic radiation is X-radiation. 